Shell solid, laterally compressed and subrectangular in shape, very inequilateral, much longer than high and wider posteriorly; slightly inequivalve, left valve somewhat overlapping the right valve on posteroventral margin. Umbones small and situated anteriorly, cardinal area long and narrow. About 28 radial ribs (25 to 33) at each valve; ribs low and flat, wider than the interstices, slightly granulated on the anterior part of left valve. Periostracum coarse, concentrically striated. Internal margins with crenulations corresponding with the external radial ribs. No byssal gape. Colour: outside of shell whitish under the blackish brown periostracum. Inner side white, often tinged pale grey at periphery.